1. Field of the Invention
The present invention generally relates to abrasive cutters to be applied to rotating downhole tools useful in creating subterranean boreholes. More specifically, the present invention is directed to a compact cutter including an interface region between the substrate and the abrasive element to promote superior impact resistance and adhesion.
2. Description of the Prior Art
Polycrystalline diamond compacts (PDC) are commonly used in oil field drilling and machine tools. A PDC is a synthetic form of diamond that is made by pressing diamond powder and cobalt onto a cemented tungsten carbide substrate. In the press, the cobalt becomes liquid and acts as a catalyst for diamond grain growth. The result is a highly abrasive, e.g. roughly 90% as abrasive as natural diamond, and environmentally resistant component which is very adaptable to drilling systems for resistant rock formations.
Although PDC is resistant to abrasion and erosion, a PDC compact cutter demonstrates several disadvantages. The main components of the PDC system, diamond and tungsten carbide, are brittle materials subject to impact fracturing. Moreover, because tungsten carbide and diamond have different coefficients of thermal expansion, there are residual stresses in a PDC system because the tungsten carbide demonstrates greater contraction during the cooling phase than that of the synthetic diamond.
As a result of the aforereferenced disadvantages, attempts have been made in the art to limit the affects by modifying the geometry at the interface between the diamond and the tungsten carbide. Such modifications have usually taken the place of an irregular, non planar interface geometry. The most beneficial resultant of the non-planar interface, defined as any design where the interface between the diamond and tungsten carbide is not a circular plane, is the redistribution of residual stresses. Redistributing residual stresses allow the PDC manufacturer to increase the diamond thickness, thereby providing increased wear resistance. An irregular interface is advantageous since brittle materials are more resistant to compressive loads than tensile loads. The existence of a flat interface causes tensile stress plane between the diamond and tungsten carbide. This plane generally defines a main failure locus for delamination of the diamond layer.
One cutter which first utilized a non-planar interface geometry was the xe2x80x9cClawxe2x80x9d cutter, so named as a result of the wear pattern of a worn cutter which looked like the remnants of claw marks. The interface of the xe2x80x9cClawxe2x80x9d cutter, when viewed in cross section, consists of a plurality of parallel ridges and grooves disposed across the diameter. The xe2x80x9cClawxe2x80x9d cutter provided advantages in the areas of wear resistance, but demonstrated a number of disadvantages which included the need to orient the cutter in order to position the parallel diamond inserts normal to the cutting surface. This required orientation of the cutter vis-a-vis the drill bit body complicates the manufacture process.
The so called xe2x80x9cring clawxe2x80x9d cutter adopted a similar design to that of the Claw cutter except that the Ring Claw included a enhanced thickness ring of synthetic diamond which bounded a series of parallel inserts which also includes diamond of an enhanced thickness. The Ring Claw cutter demonstrated improved wear resistance over the Claw cutter, but when the outer diamond ring became worn, demonstrated similar disadvantages as to the need for precise orientation vis-a-vis the work surface.
Another prior art cutter is known as the xe2x80x9ctarget cutterxe2x80x9d, and is characterized by an alternating grooves and ridges formed on the cutting face in the form of a target. The target cutter, while addressing the issue of orientation presented by the xe2x80x9cRing Claw cutter,xe2x80x9d demonstrated vulnerability to hoop stresses. Hoop stresses are created on the bounding ridges of tungsten carbide positioned interior to grooves filled with synthetic diamond. Hoop stresses are caused by uninterrupted concentric grooves and ridges in the PDC. During cooling of the PDC after pressing, the tungsten carbide ridges will contract and compress on the synthetic diamond rings disposed in the internal grooves. Such contraction simultaneously pulls the tungsten carbide substrate away from diamond disposed in external rings. These differential stresses create a tensile load between all of the internal tungsten carbide ridges and synthetic diamond disposed in all external grooves. such stresses can be severe enough to completely delaminate the synthetic diamond layer. A more common failure is the creation of stress zone in the interface, where fractures due to impact can originate.
Moreover, both the xe2x80x9cClawxe2x80x9d and the xe2x80x9ctarget cutterxe2x80x9d suffered from brazing problems associated with attempts to increase the thickness of the diamond layer. Such additional thicknesses also resulted in reduced impact resistance. In all such prior art cutters, the highest level of stress is found at the edge where cutting forces and impact forces are the highest. Thus, even thought the xe2x80x9cClawxe2x80x9d and xe2x80x9ctarget cutterxe2x80x9d incorporated a substrate to abrasive interface which included one or more grooves, the uninterrupted thickness and width of the abrasive in these grooves still gave rise to stresses which would often result in stress fracturing and ultimately the complete failure of the cutting element.
The present invention addresses the above and other disadvantages of prior cutter designs by providing a tool insert comprising a generally disc-shaped abrasive compact having a major front surface and a beveled or arcuate back surface, where at least a part of the periphery of the insert defines a cutting edge. The insert itself is comprised of a hard metal substrate backed to an abrasive compact material, e.g. synthetic diamond, where the substrate defines a partially beveled or tapered surface.
In a first embodiment, the substrate defines a major planar surface which incorporates a circumferential slot or groove at its outermost radial border such that the thickness of the abrasive layer about the slot or groove is greater than at the planar region. The circumferential groove is defined by an upper and lower inner boundary and the radial border or periphery of the element. In this embodiment, the trace between the upper and lower portions of the inner boundary is characterized by an arcuate or beveled edge.
In a second embodiment, the substrate defines a major planar surface which incorporates a circumferential slot or groove at its outermost radial border such that the thickness of the abrasive layer about the slot or groove is greater than at the planar region. The circumferential groove is again defined by an upper and lower inner boundary and the radial border or periphery of the element. In this embodiment, however, the trace between the upper and lower portions of the inner boundary includes one or more steps which may themselves include an arcuate or beveled subtrace.
The present invention offers a number of advantages over the prior art. One such advantage is the ability to increase the thickness of the cutting material where it is most needed to resist stresses experienced in the cutting processes.
The present invention also serves to minimize failures occasioned as a result of differential expansion coefficients between the abrasive material and the underlying substrate during the cooling phase. The cutter also presents a uniform thickness of abrasive material around the circumference of the cutter with relative radial symmetry.
Further, the cutter of the present invention facilitates drill bit manufacture since the cuter can be oriented at any angle on the drill bit body during assembly.
Yet other advantages of the invention will become obvious to those skilled in the art in light of the drawings and the description of the preferred embodiment.